The present invention generally relates to equipment and processes for producing tubes, and more particularly to a tube mill and process capable of in-line deposition of a braze coating on a tube, including tubes suitable for use in the manufacture of heat exchangers.
The manufacture of heat exchangers requires the joining of fluid passages (typically metal tubes) to heat transfer surfaces such as fins. For example, one type of heat exchanger construction used in the automotive industry comprises a number of parallel tubes that are joined to and between a pair of manifolds, creating a parallel flow arrangement. The ends of the tubes are typically metallurgically joined (brazed, soldered, or welded) to tube ports, generally in the form of holes or slots formed in a wall of each manifold. The tubes thermally communicate with high surface area fins in order to maximize the amount of surface area available for transferring heat between the environment and a fluid flowing through the tubes. The fins are typically in the form of flat panels having apertures through which tubes are inserted, or in the form of sinusoidal centers that are positioned between adjacent pairs of “flat” oval tubes with oblong cross-sections.
Tube-to-fin joints formed by brazing techniques are characterized by strong metallurgical bonds that can be formed at temperatures that do not exceed the softening temperatures of the components being joined, such as copper and aluminum tubes and fins widely used in automotive heat exchangers. One such brazing process is the CUPROBRAZE® process, which involves depositing a braze paste on the tubes or fins, which are then assembled and heated to a suitable brazing temperature. The paste used in the CUPROBRAZE® process contains binders and a metal braze alloy based on the CuSnNiP system, for example, about 75% copper, about 15% tin, about 5% nickel, and about 5% phosphorus. Equipment for the CUPROBRAZE® process is commercially available from various sources, such as Schöler Spezialmaschinenbau GmbH and Bondmet, Ltd., and can be an offline standalone machine or integrated into a tube mill to provide a process that continuously forms and coats tubing suitable for heat exchanger applications.
Shortcomings of brazing operations that use a braze paste include relatively high material costs, labor requirements, and inconsistent coating thickness. Therefore, alternative processes would be desirable.